INSOLUBILIZATION OF COAL SOLUBLE CONSTITUENTS IN SOME BITUMINOUS COALS BY REFLUXING WITH PYRIDINE

The solvent-treated coals were extracted with CS2-N-methyl-2-pyrrolidinone (NMP) mixed solvent at room temperature to investigate the effect of the treatment on extractability. The refluxing with pyridine greatly decreased the extraction yield compared to that of the untreated coal. The treatment with pyridine at room temperature needed a longer time, i.e., 1 week, to detect the decrease in the extraction yields of coals. A similar treatment with CS2-pyridine mixed solvent at room temperature, which is a better solvent than pyridine for coal molecules, for 1 day at room temperature, decreased the extraction yield to a great degree compared to that with pyridine. Fractionation of the soluble fractions by acetone and pyridine clarified that this decrease in the yields is due to the insolubilization of the heavy fraction, i.e., the pyridine-insoluble (E-PI) fraction in the extract. The swelling ratios of the E-PI fraction in benzene and methanol were decreased by this treatment, indicating the increase of cross-linking density. Spin concentrations of ESR spectra of the E-PI fractions were increased by refluxing with pyridine. Insolubilization mechanisms are discussed for the formation of covalent and/or noncovalent bonds between the heavy E-PI molecule and other heavy molecules (E-PI or residue) during the treatment.